smr: Load to accept pointers to const pointers

Pointers passed to the smr_entered_load() and smr_serialized_load()
macros are in the end used as arguments to atomic_load_*ptr(), so
convert them to the now acceptable 'const uintptr_t *' ones (instead of
'uintptr_tr *'), making these macros accept pointers to constant
pointers.

Reviewed by: kib
MFC after: 4 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48497

(cherry picked from commit 1ac0afaa962bc847294d5e6bf1e749b7ffa78cfd)